A teacher is someone who teaches. In the case of Go, it's just someone who teaches Go. =) Also known as sensei in Japanese.
"Webster's Revised Unabridged Dictionary (1913)"
Teacher Teach"er, n.
1. One who teaches or instructs; one whose business or occupation is to instruct others; an instructor; a tutor.
Several teachers give online lesson. See e.g. KGS Teachers.